A computer system may use a television system as an output display device. For the purposes of this application, the term "television" may include, but is not limited to, NTSC, PAL or SECAM televisions, receivers, and monitors, or the like. In addition, as used herein, the term "television" may also refer to other types or formats of television displays including HDTV, either analog (e.g., MUSE) or digital.
Such a computer system may however be designed to generate an output having different characteristics than an input required by a television system. For example, a computer system implementing a VGA standard may generate an output image of 640.times.480 pixels at a rate of 72 frames per second. On the other hand, a television system implementing NTSC standard may have 525 horizonal lines divided into even and odd interlaced fields of 262.5 lines each as shown in FIG. 1A.
FIG. 1A depicts a frame of an NTSC image (or other interlaced image). Each frame of the NTSC image is comprised of interlaced even and odd fields. FIG. 1B depicts field one, the odd field which comprises odd numbered scan lines. FIG. 1C depicts field two, an even field which comprises even numbered scan lines. The two fields are interlaced to produce the composite frame in FIG. 1A. Each frame of FIG. 1A may be refreshed at a pre-determined frame rate. For an NTSC image such frame rate may be 30 Hz. The fields may be refreshed at a field rate twice the frame rate, which for an NTSC image would be 60 Hz.
Of the 525 horizontal lines in an NTSC video signal, some may be taken up with what is known as the Vertical Blanking Interval (VBI) which includes pre-equalizing pulses (occupying three horizontal lines), the vertical sync pulse interval (occupying three horizontal lines), post-equalizing pulses (occupying three horizontal lines), and ten to fourteen non-video lines, which may be used for reference sub-carrier phase, close captioning, descrambling data, or other so-called "in-band" data. As a result, of the 525 horizontal lines in an NTSC video signal, only 480 may be available for active video.
These 480 lines of active video could be used to display 480 lines of pixel data from a 640 by 480 pixel resolution VGA display with relative ease. However, a television, unlike a computer monitor, uses a technique known as overscan to insure that the picture fills the entire video display (e.g., picture tube). Unfortunately, this overscan technique, when applied to the output of a computer system may result in a display image having truncated upper and lower portions as well as truncated left and right portions. For a computer display image, such truncation is not acceptable, as useful information may appear in the truncated portions of the display image.
Thus, in order to display a high resolution computer system (Ex. VGA, SVGA) video display on a television, the vertical resolution of the display may be decreased to accommodate the limitation of the television. For most commercially sold televisions, vertical overscan rarely exceeds 15%. Thus, if the computer system output display image could be adapted to fit within 400 lines (For an NTSC television), all of the display image may appear on a television system without being truncated due to overscan. As a computer system output display may include more horizontal lines (for example 480 horizontal lines in the case of VGA standard, and 768 in the case of SVGA standard), there is a need to filter the horizontal lines to downscale the computer system output display.
An additional problem is presented when attempting to display a high resolution computer system video output on an interlaced video display such as a television. High resolution computer systems (e.g., VGA, SVGA or the like) may store image data as individual pixel data in non-interlaced form in a video memory (VMEM). The pixel data may be scanned out of the video memory in sequence, converted into analog form, and displayed on a VGA compatible monitor. In order to generate a display image without flicker, a VGA compatible monitor may be refreshed at a relatively high refresh rate (e.g., 60 or 75 Hz) in order to take advantage of the persistence of vision phenomenon.
However, an interlaced video display such as a television may be refreshed at a lower frame rate, for example, 30 Hz. For a typical analog television signal, with relatively low resolution and contrast, such a refresh rate is acceptable for viewing without perceptible flicker. However, a computer display image such as a graphics display image may include a high contrast image having a horizontal line or edge which is one pixel (i.e., one horizontal line) in height. Such a line may only appear on one of two interlaced (even and odd) fields and thus may be refreshed only once per frame. For the viewer, such lines or edges may have a perceptible flicker which may make viewing the image annoying and disorienting.
In general, it has been noted that such flicker effects are usually found at horizontal edges or in thin horizontal lines where the intensity or color of the line or edge is substantially different from an adjacent line in the opposite field (i.e., high contrast areas). For example, a black horizontal line on a white background may exhibit such a flicker effect. One technique for reducing flicker involves averaging or otherwise combining each line of an interlaced display with a portion of one or more adjacent lines so as to reduce the apparent contrast between adjacent lines and thus reduce flicker.
A prior computer system may include additional circuitry to combine adjacent horizontal lines of the computer system output image and to downscale the computer system output image for display on a television. Such downscaling may be required for the television to avoid truncation of output image as explained above or to otherwise fit a computer system output image of a high resolution in a field (shown in FIGS. 1B and 1C). The additional circuitry may either modify a pre-existing circuit (for example, a circuit that generates VGA or SVGA compatible output image) or be implemented as a separate module that modifies an output of such pre-existing circuit.
The design of such an additional circuitry may be complex as the number of horizontal lines in a computer system output image may not be an integral multiple of number of horizontal lines in a frame of a television display. For example, displaying a VGA output image on an NTSC television may require downscaling from 480 horizontal lines to 400 lines per television frame as explained above.
A prior art computer system may drop (or ignore) one out of every six horizontal VGA lines, and generate five horizontal lines for television display by combining the remaining five VGA horizontal lines. The computer system may drop horizontal VGA lines as the television may require input at a pre-determined constant rate, for example one line for every 63.56 micro seconds in the case of an NTSC television.
One problem with such a prior art system is that display information may be lost due to line dropping. For example, a spread sheet/graphics/CAD application may generate a horizontal line of one pixel height (i.e., one horizontal line), and that horizontal line may be dropped. Line dropping may also result in perceptible flicker as the information corresponding to dropped lines may be refreshed only in some frames. The flicker problem and information loss problem may be compounded due to the high contrast generally present in computer displays. Due to such problems, the television display quality may be unacceptable.